1. Field of the Invention
This invention relates to the field of meters and particularly to the field of fluid flowmeters.
2. Description of the Prior Art
Gas flowmeters presently used to measure the consumption of natural gas are remarkably effective and reliable instruments. Nevertheless, improvements are desirable, particularly in view of rising demands for and cost of energy in all forms, including natural gas.
Gas meters for domestic use are generally of a mechanical type which utilize the displacement of a known volume by the flowing gas to determine the gas consumption. In such meters, an elastomeric bag can be filled and emptied or a chamber can be filled and emptied by a rotating mechanism or piston. Such prior art gas meters provide accurate measurements at flow rates ranging from less than 1.0 to more than 200 cfh and at pressure drops less than 0.05 psi during maximum flow. Actual accuracy is of the order of 0.5% over most of the flow rate range, dropping to serveral percent at the low flow rate end of the range. Improved accuracy is desirable over the entire range, particularly the low flow rate end.
Gas meters for other uses include types which determine flow rate by measuring the pressure drop across a fixed impedance (or orifice) and integrating this measurement over time. However, such orifice meters are not practical for measuring flow in applications such as domestic gas meters because the flow rates vary over wide limits. For example, in fixed-impedance turbulent flow type orifice meters, the pressure drop through the orifice is proportional to the square of the flow rate. A 0.5 to 200 cfh variation in residential flow rate (a factor of 400) would require a factor of 1.6.times.10.sup.5 variation in the measurement of pressure differential. Such measurements cannot be made with suitable accuracy under domestic operating conditions.
When a porous plug is used in a fixed-orifice type meter to change the flow from turbulent to laminar, the pressure drop is proportional to the first power of the flow rate. This would make the pressure measurement less critical but raise the problem of orifice plugging by impurities in the gas stream.
Another class of flowmeter is known which utilizes a variable orifice. One such flowmeter (a rotameter) utilizes a tapered glass tube with a moving ball. In such meters, the position of the moveable member, rather than the pressure drop across the orifice, is used to indicate the flow rate. The flow rate is determined by the position of the ball within the glass tube as described in U.S. Pat. No. 2,778,223.